In order to provide an optical recording medium that can record and reproduce at a blue laser wavelength or lower, the development of a blue laser that enables extra high density recording rapidly proceeds, and an optical recording medium compatible with it is developed.
In conventional WORM optical recording media, a recording layer including an organic material is irradiated with laser light to cause a change in refractive index mainly due to decomposition or transformation of the organic material to form recording pits, and the optical constant and decomposition behavior of the organic material used in the recording layer are important factors for forming good recording pits.
Therefore, a material that has optical properties and decomposition behavior suitable for a blue laser needs to be selected as the organic material used in the recording layer of a blue laser compatible WORM optical recording medium.
Conventional WORM optical recording media having high to low polarity are so configured that high reflectance is ensured during non-recorded state and that the organic material is decomposed by laser irradiation to cause a large change in refractive index, thereby a large degree of modulation is obtained, so that the recording and reproducing wavelength is selected to be located at the end of a large absorption band on the long wavelength side.
This is because the end of the large absorption band of the organic material on the long wavelength side is a wavelength region that has a moderate absorption coefficient and provides a large refractive index.
However, a recording material that has the value of optical properties for a blue laser comparable to that of conventional red laser compatible recording materials is not found. This is because, in order to have the absorption band of an organic material near the wavelength of a blue laser, the molecular skeleton needs to be made small, or the conjugated system needs to be shortened, but if so, a decrease in absorption coefficient, that is, a decrease in refractive index, is caused. In other words, many organic materials that have an absorption band near the wavelength of a blue laser exist, and the absorption coefficient can be controlled, but they do not have a large refractive index, so that a large degree of modulation cannot be obtained.
Also, organic dye materials have a poorer stability than inorganic materials, so that they have problems in the storage property and light resistance. Then, the use of an inorganic material in the recording layer is studied for WORM optical recording media compatible with a blue laser.
As a recording layer for a WORM optical recording medium compatible with a blue laser, for example, one using a phase change material similar to that of a rewritable optical recording medium is proposed in Patent Literature 1, but a WORM optical recording medium requires long period storage, and the phase change material has an insufficient storage property.
A method, in which a plurality of layers of inorganic material are laminated and their reaction is used for recording, is also proposed in Patent Literature 2, but one using reaction of a plurality of layers is not suitable for long period storage, because the reaction proceeds with time.
In this connection, the present inventors previously proposed the usefulness of a recording layer that has as a main component an oxide of a metal or semimetal, particularly bismuth oxide, as a WORM optical recording medium that is capable of high density recording even with laser light having a blue wavelength (see Patent Literatures 3 to 6).
On the other hand, there are Patent Literatures 7 and 8 as technologies that are similar to the above previous applications of the present inventors: a recording layer in which Te, O, and further another element are added is disclosed in Patent Literature 7; and one using an incomplete oxide of transition metal is disclosed in Patent Literature 8. In the above Patent Literature 8, although it is assumed that one including an element other than transition metal is also included, however, no specific element other than Al is described, the definition of transition metal is unclear because Zn, Y, and the like may or may not be included, and no detail description other than W and Mo is provided.
Further, there is no specific description of a problem to be solved by the invention, that is, higher sensitivity, in these Patent Literatures 7 and 8.
It can be said that a WORM optical recording medium using oxide for a recording layer is suitable for higher density, because the heat conductivity of the recording layer is low, so that heat interference between recording marks can be suppressed.
When oxide is used for the recording layer, decreasing the degree of oxidation of oxide (increasing the amount of oxygen deficiency) is proposed as a method for further improving the recording property.
Technologies using a material, in which the amount of oxygen is smaller than that of the stoichiometric composition, in the red and infrared wavelength regions include, for example, one using TeOx (0<x<2) (see Patent Literature 9), one including at least one selected from TeOx, GeOx, SnOx, BiOx, SbOx, and TlOx, and at least one of S and Se (see Patent Literature 10), one containing Te and Sb in low oxide GeOx or containing Te and Ge in SbOx (see Patent Literature 11), one using Ni-low oxide expressed by NiOx (see Patent Literature 12), an information recording mode in which In-low oxide is irradiated with laser light to form an image (see Patent Literature 13), and the like.
Also, an invention that relates to low oxide in the red wavelength region and in which an element selected from Sn, In, Bi, Zn, Al, Cu, Ge, and Sb is added to TeOx is disclosed in Patent Literature 14. There is also description on BiOx in the text, and it is described to be effective when Te, Sb, or Ge is added. However, this Patent Literature 14 is an invention using a so-called blackening phenomenon in which the transmittance of light is changed by light irradiation, and an invention of a film having reversibility in which the transmittance of one recorded by blackening is returned to the original transmittance again by light irradiation.
However, there is no description regarding the effect of BiOx in the blue region or no detail description other than a system including TeOx in the above Patent Literature 14.
Further, in all the above Patent Literatures 9 to 14, recording and reproducing are performed in the red or infrared wavelength region, and they are not technologies compatible with a blue laser.
As a technology that relates to a WORM optical recording medium compatible with a blue laser and decreases the degree of oxidation of oxide used as a recording layer (increases the amount of oxygen deficiency), the present inventors propose a WORM optical recording medium including a recording layer including at least Bi oxide and an oxide of M (M is at least one element selected from Mg, Al, Zn, Li, Si, Hf, Sn, Y. and B) as main components on a substrate, wherein the oxygen content in the oxide is smaller than that of the stoichiometric composition (see Patent Literature 15).
In this Patent Literature 15, an additive, which is added to Bi oxide, is contained as oxide, and this method is very effective for improving the recording and reproducing properties of a Bi type recording material, but it cannot be said that the recording sensitivity of the recording layer is sufficient when a further increase in recording speed predicted in the future is considered.
Thus, in related art literatures, there is no disclosure nor suggestion of a technology in which Bi oxide, and a simple substance of each of one or more elements M that enhance a light absorption function for a recording and reproducing laser light are contained in a recording layer, as in the present invention, in order to further improve the recording sensitivity.
Also, the present applicant discloses in Patent Literature 16 and the like that a WORM optical recording medium having a recording layer including Bi, B, and O (oxygen) exhibits good properties, and it can be confirmed to exhibit very excellent recording and reproducing properties.
Besides the above, a WORM optical recording medium having a recording layer including Bi oxide as a main component is also disclosed in Patent Literature 17, however, a system to which carbon and nitrogen are added is not discussed.
Also, although an information recording medium using a recording layer including metal nitride and metal carbide is disclosed in Patent Literatures 18 to 19, it includes metal nitride as a main component, and decomposition of metal nitride is the recording principle, so that it is not reference for the present invention including Bi and O (oxygen) as main components.
On the other hand, there is a trend toward higher density and higher speed in optical discs, and also for conventional DVDs, higher density is achieved by two-layer configuration, and also for higher speed, even media capable of 16×-speed recording have appeared. This trend is also the same in optical discs using a blue LD, and is considered to be directed toward high speed recording, and the development of optical recording media for high speed recording is beginning.
However, the related art does not discuss high speed recording at all. Also, the invention of Patent Literature 17 aims at an improvement in recording and reproducing properties and reliability (reproduction stability, storage stability, and the like), and does not discuss high speed recording. Further, it discusses materials in which various elements X are added to bismuth oxide, but C and N are not included in X, and no specific example in which two or more elements are added to bismuth oxide is illustrated.
Separately, the present inventors propose a target including Bi and Fe as a sputtering target for forming a recording layer including Bi oxide as a main component in a WORM optical recording medium that is capable of high density recording with laser light in the blue wavelength region (350 nm to 500 nm) (see Patent Literature 20).
The sputtering method is widely known as one of gas-phase formation methods for a thin film and is also used in industrial thin film manufacture. In the sputtering method, a film is formed by preparing a target material including the same component as the component of the intended film, and usually, colliding Ar (argon) gas ions generated by glow discharge against this target material to knock out the constituent atoms of the target material to deposit the atoms on a substrate. Particularly, oxide generally has a high melting point, so that a method, such as a vapor deposition method, is not preferable, and high frequency sputtering in which high frequency is applied is often used.
The sputtering method is actually often used in the manufacture process and is also advantageous in throughput. However, when a film including a mixed material of two or more elements is formed, the composition of the target and the composition of the film are often not the same, so that the composition of the target needs to be studied. The structure and nature of the film often differ depending on the form of a compound constituting the target, so that this point also needs to be studied. From the viewpoint of the production cost, a further improvement in the speed of film deposition is also necessary. For an improvement in the speed of film deposition, larger electric power needs to be input, and also in that case, an improvement in the strength of the target is necessary such that the target is not broken.
Besides the above, a method for forming a recording layer containing carbon and Bi, by sputtering in a mixed gas of CH4 or the like and Ar, using a low melting point metal target, such as Bi, is disclosed in Patent Literature 21, but it differs from the present invention including Bi and oxygen as main components.
In an optical recording medium, a highly sensitive recording property in which recording is possible at a low recording power is required in terms of the limit of the power of laser light, durability, power saving performance, and the like. Also, high linear velocity recording for an improvement in the speed of information transfer, with high density recording, are required, but in that case, high linear velocity recording requires a further improvement in recording sensitivity, compared with low linear velocity recording. Also, for compatibility, the optical recording medium needs to be compatible with the entire range of recording linear velocity from low linear velocity to high linear velocity.    [Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 09-286174    [Patent Literature 2] Japanese Patent Application Laid-Open (JP-A) No. 2004-79020    [Patent Literature 3] Japanese Patent Application Laid-Open (JP-A) No. 2005-161831    [Patent Literature 4] Japanese Patent Application Laid-Open (JP-A) No. 2005-108396    [Patent Literature 5] Japanese Patent Application Laid-Open (JP-A) No. 2003-48375    [Patent Literature 6] Japanese Patent Application Laid-Open (JP-A) No. 2006-116948    [Patent Literature 7] Japanese Patent Application Laid-Open (JP-A) No. 2002-133712    [Patent Literature 8] Japanese Patent Application Laid-Open (JP-A) No. 2003-237242    [Patent Literature 9] Japanese Patent Application Laid-Open (JP-A) No. 50-46317    [Patent Literature 10] Japanese Patent (JP-B) No. 1444471    [Patent Literature 11] Japanese Patent (JP-B) No. 1849839    [Patent Literature 12] Japanese Patent (JP-B) No. 2656296    [Patent Literature 13] Japanese Patent Application Laid-Open (JP-A) No. 51-21780    [Patent Literature 14] Japanese Patent Application Publication (JP-B) No. 7-25209    [Patent Literature 15] Japanese Patent Application Laid-Open (JP-A) No. 2006-248177    [Patent Literature 16] Japanese Patent Application Laid-Open (JP-A) No. 2006-247897    [Patent Literature 17] Japanese Patent (JP-B) No. 3802040    [Patent Literature 18] Japanese Patent Application Laid-Open (JP-A) No. 2006-182030    [Patent Literature 19] Japanese Patent (JP-B) No. 3810076    [Patent Literature 20] Japanese Patent Application Laid-Open (JP-A) No. 2007-169779    [Patent Literature 21] Japanese Patent (JP-B) No. 1480945